Active matrix device and method of driving

ABSTRACT

An active matrix electro-wetting on dielectric (AM-EWOD) device includes a plurality of array elements arranged in an array, each array element including array element circuitry, an element electrode, and a reference electrode. The array element circuitry includes an actuation circuit configured to apply actuation voltages to the electrodes, and an impedance sensor circuit configured to sense impedance at the array element electrode to determine a droplet property. The actuation circuitry includes a memory capacitor for storing voltage data corresponding to either an actuated state or an unactuated state of the array element, and an input applied to the memory capacitor operates to effect an operation of the impedance sensor circuit. Such input may isolate the array element from the actuation voltage during operation of the impedance sensor circuit, and the memory capacitor may operate as part of the impedance sensor circuit as a reference capacitor for determining the droplet property.

TECHNICAL FIELD

The present invention relates to active matrix arrays and elementsthereof. In a particular aspect, the present invention relates todigital microfluidics, and more specifically to Active MatrixElectro-wetting-On-Dielectric (AM-EWOD), and further relates to methodsof driving such a device.

BACKGROUND ART

Electro-wetting on dielectric (EWOD) is a well known technique formanipulating droplets of fluid by application of an electric field.Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in anactive matrix array incorporating transistors, for example by using thinfilm transistors (TFTs). It is thus a candidate technology for digitalmicrofluidics for lab-on-a-chip technology. An introduction to the basicprinciples of the technology can be found in “Digital microfluidics: isa true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007)3:245-281).

FIG. 1 shows a part of a conventional EWOD device in cross section. Thedevice includes a lower substrate 72, the uppermost layer of which isformed from a conductive material which is patterned so that a pluralityof array element electrodes 38 (e.g., 38A and 38B in FIG. 1) arerealized. The electrode of a given array element may be termed the arrayelement electrode 38. The liquid droplet 4, comprising a polar material(which is commonly also aqueous and/or ionic), is constrained in a planebetween the lower substrate 72 and a top substrate 36. A suitable gapbetween the two substrates may be realized by means of a spacer 32, anda non-polar fluid 34 (e.g. oil) may be used to occupy the volume notoccupied by the liquid droplet 4. An insulator layer 20 disposed uponthe lower substrate 72 separates the conductive element electrodes 38A,38B from a first hydrophobic coating 16 upon which the liquid droplet 4sits with a contact angle 6 represented by θ. The hydrophobic coating isformed from a hydrophobic material (commonly, but not necessarily, afluoropolymer).

On the top substrate 36 is a second hydrophobic coating 26 with whichthe liquid droplet 4 may come into contact. Interposed between the topsubstrate 36 and the second hydrophobic coating 26 is a referenceelectrode 28.

The contact angle θ 6 is defined as shown in FIG. 1, and is determinedby the balancing of the surface tension components between thesolid-liquid (γ_(SL)), liquid-gas (γ_(LG)) and non-polar fluid (γ_(SG))interfaces, and in the case where no voltages are applied satisfiesYoung's law, the equation being given by:

$\begin{matrix}{{\cos\;\theta} = \frac{\gamma_{SG} - \gamma_{SL}}{\gamma_{LG}}} & \left( {{equation}\mspace{14mu} 1} \right)\end{matrix}$In certain cases, the relative surface tensions of the materialsinvolved (i.e the values of γ_(SL), γ_(LG) and γ_(SG)) may be such thatthe right hand side of equation (1) is less than −1. This may commonlyoccur in the case in which the non-polar fluid 34 is oil. Under theseconditions, the liquid droplet 4 may lose contact with the hydrophobiccoatings 16 and 26, and a thin layer of the non-polar fluid 34 (oil) maybe formed between the liquid droplet 4 and the hydrophobic coatings 16and 26.

In operation, voltages termed the EW drive voltages, (e.g. V_(T), V₀ andV₀₀ in FIG. 1) may be externally applied to different electrodes (e.g.reference electrode 28, array element electrodes 38, 38A and 38B,respectively). The resulting electrical forces that are set upeffectively control the hydrophobicity of the hydrophobic coating 16. Byarranging for different EW drive voltages (e.g. V₀ and V₀₀) to beapplied to different element electrodes (e.g. 38A and 38B), the liquiddroplet 4 may be moved in the lateral plane between the two substrates72 and 36.

In the following description, it will be assumed that an element of anEWOD device, such as the device of FIG. 1, may receive “digital” data sothat the element is required to be put in either an “actuated” state inwhich the voltage applied across the element is sufficient for a liquiddroplet in the element (if one is present in the element) to experiencea significant electro-wetting force, or a “non-actuated” state in whichthe voltage applied across the element is not sufficient for a liquiddroplet in the element (if one is present in the element) to experiencea significant electro-wetting force. An element of an EWOD device may beput into the actuated state by applying a voltage difference across theEWOD element having a magnitude that is equal to, or greater than, athreshold voltage V_(EW), whereas if the voltage difference across theEWOD element has a magnitude that is less than the threshold voltageV_(EW) the element is in its non-actuated state. The threshold voltageV_(EW) is often referred to as an “actuation voltage”, and this term isused below. In practice, the threshold voltage may typically bedetermined as the minimum voltage required to effect droplet operations,for example the moving or splitting of droplets. In practice, thenon-actuated state may typically be zero volts. Typically EWOD systemsmay be considered to be digital, in that the EWOD elements areprogrammed either to an actuated or non-actuated state. It shouldhowever be understood that an EWOD device may also be operated bysupplying analogue data, such that EWOD elements may be partiallyactuated.

U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses apassive matrix EWOD device for moving droplets through an array.

U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) disclosesa two dimensional EWOD array to control the position and movement ofdroplets in two dimensions.

U.S. Pat. No. 6,565,727 further discloses methods for other dropletoperations including the splitting and merging of droplets, and themixing together of droplets of different materials.

U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007)describes how TFT based thin film electronics may be used to control theaddressing of voltage pulses to an EWOD array by using circuitarrangements similar to those employed in Active Matrix (AM) displaytechnologies.

The approach of U.S. Pat. No. 7,163,612 may be termed “Active MatrixElectro-wetting on Dielectric” (AM-EWOD). There are several advantagesin using TFT based thin film electronics to control an EWOD array,namely:

-   -   Electronic driver circuits can be integrated onto the lower        substrate 72.    -   TFT-based thin film electronics are well suited to the AM-EWOD        application. They are cheap to produce so that relatively large        substrate areas can be produced at relatively low cost.    -   TFTs fabricated in standard processes can be designed to operate        at much higher voltages than transistors fabricated in standard        CMOS processes. This is significant since many EWOD technologies        require EWOD actuation voltages in excess of 20V to be applied.

A disadvantage of U.S. Pat. No. 7,163,612 is that it does not discloseany circuit embodiments for realizing the TFT backplane of the AM-EWOD.

EP2404675 (Hadwen et al., published Jan. 11, 2012) describes arrayelement circuits for an AM-EWOD device. Various methods are known forprogramming and applying an EWOD actuation voltage to the EWOD elementelectrode. The programming function described includes a memory elementof standard means, for example, based on Dynamic RAM (DRAM) or StaticRAM (SRAM) and input lines for programming the array element.

Whilst EWOD (and AM-EWOD) devices can be operated with either DC or ACactuation voltages, in practice there are many reasons for preferring anAC method of driving, as reviewed in the previously cited reference R.B. Fair, Microfluid Nanofluid (2007) 3:245-281). It may be noted thatdroplets can be actuated and manipulated for a wide range of AC drivingfrequencies ranging typically from a few hertz to several kHz.

U.S. Pat. No. 8,653,832 (Hadwen et al., issued Feb. 18, 2014) describeshow an impedance (capacitance) sensing function can be incorporated intothe array element. The impedance sensor may be used for determining thepresence and size of liquid droplets present at each electrode in thearray.

UK Application GB1500261.1, which is herein incorporated by reference,describes a two transistor (2T) array element circuit and a method ofdriving for implementing an AC driving method of driving. The 2T arrayelement actuation circuit disclosed is shown in FIG. 2 of the currentapplication. This UK application further includes an embodiment showinghow the impedance (capacitance) sensing function of U.S. Pat. No.8,653,832 can be combined with the 2T array element actuation circuit.The array element circuit including the sensor function is shown in FIG.3 and contains a total of 5 transistors, 3 capacitors and 9 addressinglines. Addressing lines DATA and ENABLE control access to a Dynamic RAMmemory circuit comprising the transistor to which they are connected anda capacitor. The voltage programmed to this capacitor in turn controlswhether or not the input signal ACTUATE is connected through to an arrayelement electrode. The input signal SEN may further be used to isolatethe element electrode from the ACTUATE signal when the sensor is beingoperated. The sensor function is controlled by two voltage signalsapplied to terminals RWS and RST. The voltage signal applied to RSTresets the voltage at the gate of a sense transistor (connected betweenVDD and COL) to a reset potential VRST. The voltage signal applied toRWS perturbs the voltage at the element electrode by an amount dependenton the ratio of the fixed capacitors in the circuit present at theelement electrode and the capacitance presented by the presence orabsence by a liquid droplet at the element electrode. A voltage signalis thus coupled to the gate of the sensing transistor which is convertedto an output current through COL. The impedance presented at the elementelectrode may thus be measured.

SUMMARY OF INVENTION

A first aspect of the invention provides an array element circuit andmethod of driving an element of an active matrix electro-wetting ondielectric (AM-EWOD) device, the AM-EWOD element having an elementelectrode and a reference electrode.

The method comprises actuation of a liquid droplet which may be presentat the location of the array element and also the sensing of theimpedance associated with a droplet, or the absence of a droplet, at thelocation of the array element.

The method of droplet actuation is comprised of applying a firstalternating voltage to the reference electrode; and either applying tothe element electrode a second alternating voltage that has the samefrequency as the first alternating voltage and that is out of phase withthe first alternating voltage or holding the element electrode in a highimpedance state.

The “high impedance state” refers to an impedance between the elementelectrode and ground that is of the order of at least 100 Mega-ohm. The“high impedance state” may have an impedance between the elementelectrode and ground that is of the order of at least 1 Giga-ohm.

When the second alternating voltage is applied to the element electrode,the element is put in an actuated state in which the element isconfigured to actuate any liquid droplet present in the element, whilewhen the element electrode is held in the high impedance state theelement is put in a non-actuated state.

The array element circuit contains a memory function for storingprogrammed data in accordance with which the array element is configuredinto an actuated or non-actuated state. The memory function may comprisea dynamic RAM circuit, whereby the programmed information is stored as avoltage programmed onto a storage capacitor.

The method of sensing the impedance may comprise applying a voltagesignal so as to perturb the potential of the element electrode, theperturbation being a function of the impedance presented at the elementelectrode. The method of sensing the impedance may comprise comparingthe impedance presented at the element electrode with a referenceimpedance in the array element circuit.

The storage capacitor (used for storing the programmed actuation state)may further participate in the sensing function of the array elementcircuit.

According to a first aspect of the invention, the storage capacitor mayparticipate in the sensing function by perturbing the bias at a node ofthe circuit, in accordance with the application of a voltage signal.Further according to this aspect of the invention, the storage capacitormay effect the perturbing of a bias such as to disconnect an actuationvoltage signal from the element electrode during the operation of thesensor circuit.

According to a second aspect of the invention, the storage capacitor mayparticipate in the sensing function by acting as a reference impedancewith which the impedance present at the element electrode may becompared during the operation of the sensor circuit.

The AM-EWOD device may comprise a plurality of AM-EWOD elements arrangedin a matrix of rows and columns, and wherein the method may comprisearranging for an instantaneous value of the second alternating voltageapplied to a row of AM-EWOD elements to be equal to an instantaneousvalue of the first alternating voltage at a time of putting the elementelectrodes of AM-EWOD elements of the row into the high impedance state.

The advantages of the invention include:

-   -   By driving the AM-EWOD device in this way, AC electro-wetting is        achieved, with the electro-wetting voltage being switched        between +V_(EW) and −V_(EW) whilst the transistors in the array        element circuit are only required to switch a maximum voltage of        V_(EW).    -   This method of driving may be implemented in circuitry requiring        a minimal number of transistors (embodiments disclosed include a        2-Transistor array element circuit). Advantages of small array        element circuits are:        -   The size of the array element is minimized. This in turn            facilitates larger format arrays, and also the manipulation            of smaller liquid droplets.        -   Smaller/simpler circuits generally facilitate higher            manufacturing yield.    -   Smaller/simpler circuits may facilitate a device arrangement        that has a higher optical transparency, the thin film        electronics being only partially transparent. Optical        transparency may be important in performing chemical tests which        involve a change in the optical properties of the liquid droplet        which is then measured.    -   Embodiments of the invention can be realized requiring only        n-type (or only p-type) transistors in the array element        circuit. The AM-EWOD device can thus be fabricated in a single        channel transistor manufacturing process.

BRIEF DESCRIPTION OF DRAWINGS

In the annexed drawings, like references indicate like parts orfeatures:

FIG. 1 shows prior art, particularly a schematic diagram depicting aconventional EWOD device in cross-section;

FIG. 2 shows prior art, particularly a 2-transistor AM-EWOD arrayelement actuation circuit

FIG. 3 shows prior art, particularly a 2-transistor AM-EWOD arrayelement actuation circuit with additional impedance sensor function;

FIG. 4 is a schematic diagram depicting a an AM-EWOD device in schematicperspective in accordance with a first and exemplary embodiment of theinvention;

FIG. 5 shows a cross section through some of the array elements of theexemplary AM-EWOD device of FIG. 4;

FIG. 6A shows a circuit representation of the electrical load presentedat the element electrode when a liquid droplet is present;

FIG. 6B shows a circuit representation of the electrical load presentedat the element electrode when no liquid droplet is present;

FIG. 7 is a schematic diagram depicting the arrangement of thin filmelectronics in the exemplary AM-EWOD device of FIG. 4 according to afirst embodiment of the invention;

FIG. 8 shows a schematic arrangement of the array element circuit inaccordance with a first embodiment of the invention;

FIG. 9 is a schematic diagram depicting the array element circuit foruse in the array elements of the exemplary AM-EWOD device of FIG. 4according to a first embodiment of the invention;

FIG. 10 is a timing diagram showing the timings of voltage pulses V1 andV2 according to a first embodiment of the invention;

FIG. 11 is a timing diagram showing an exemplary arrangement of thetiming signals for driving the array elements of the exemplary AM-EWODdevice of FIG. 4 according to a first embodiment of the invention;

FIG. 12 is a schematic diagram depicting the array element circuit foruse in the array elements of the exemplary AM-EWOD device of FIG. 4according to a second and exemplary embodiment of the invention;

FIG. 13 is a timing diagram showing an exemplary arrangement of thetiming signals for driving the array elements of the exemplary AM-EWODdevice of FIG. 4 according to a second embodiment of the invention;

FIG. 14 is a schematic diagram depicting the array element circuit foruse in the array elements of the exemplary AM-EWOD device of FIG. 4according to a third and exemplary embodiment of the invention;

FIG. 15 is a timing diagram showing an exemplary arrangement of thetiming signals for driving the array elements of the exemplary AM-EWODdevice of FIG. 4 according to a third embodiment of the invention;

FIG. 16 is a schematic diagram depicting the array element circuit foruse in the array elements of the exemplary AM-EWOD device of FIG. 4according to a fourth and exemplary embodiment of the invention;

FIG. 17 is a timing diagram showing an exemplary arrangement of thetiming signals for driving the array elements of the exemplary AM-EWODdevice of FIG. 4 according to a fourth embodiment of the invention;

FIG. 18 is a schematic diagram depicting the array element circuit foruse in the array elements of the exemplary AM-EWOD device of FIG. 4according to a fifth and exemplary embodiment of the invention;

FIG. 19 is a timing diagram showing an exemplary arrangement of thetiming signals for driving the array elements of the exemplary AM-EWODdevice of FIG. 4 according to a fifth embodiment of the invention;

FIG. 20 is a schematic diagram depicting the array element circuit foruse in the array elements of the exemplary AM-EWOD device of FIG. 4according to a sixth and exemplary embodiment of the invention;

FIG. 21 is a timing diagram showing an exemplary arrangement of thetiming signals for driving the array elements of the exemplary AM-EWODdevice of FIG. 4 according to a sixth embodiment of the invention.

DESCRIPTION OF REFERENCE NUMERALS

-   -   4 liquid droplet    -   6 contact angle θ    -   16 First hydrophobic coating    -   20 Insulator layer    -   26 Second hydrophobic coating    -   28 Reference electrode    -   32 Spacer    -   34 Non-polar fluid    -   36 Top substrate    -   38/38A and 38B Array Element Electrodes    -   40/40A/40B Electrical load    -   42 Electrode array    -   44 Reference capacitor    -   46 Actuation circuit    -   48 Sensor circuit    -   52 Transistor    -   54 Transistor    -   56 Memory capacitor    -   58 Transistor    -   60 Sensor capacitor    -   62 Sensing Transistor    -   64 Transistor    -   66 Transistor    -   72 Lower Substrate    -   74 Thin film electronics    -   76 Row driver circuit    -   78 Column driver circuit    -   80 Serial interface    -   82 Connecting wires    -   83 Voltage supply interface    -   84 Array element circuit    -   86 Column detection circuit    -   88 Sensor row addressing

DETAILED DESCRIPTION OF INVENTION

FIG. 4 is a schematic diagram depicting an AM-EWOD device in accordancewith an exemplary embodiment of the present invention. The AM-EWODdevice has a lower substrate 72 with thin film electronics 74 disposedupon the lower substrate 72. The thin film electronics 74 are arrangedto drive the array element electrodes 38. A plurality of array elementelectrodes 38 are arranged in an electrode array 42, having X by Yelements where X and Y may be any integer. A liquid droplet 4, which maycomprise any polar liquid and which typically may be ionic and/oraqueous in nature, is enclosed between the lower substrate 72 and a topsubstrate 36, although it will be appreciated that multiple liquiddroplets 4 can be present. A non-polar fluid 34 is used to fill thespace between the substrates and may comprise an oil (for examplen-dodecane, silicone oil or other alkane oil) or may be air.

FIG. 5 is a schematic diagram depicting a pair of the array elementelectrodes 38A and 38B in cross section that may be utilized in theAM-EWOD device of FIG. 4. The device configuration is similar to theconventional configuration shown in FIG. 1, with the AM-EWOD devicefurther incorporating the thin-film electronics 74 disposed on the lowersubstrate 72. The uppermost layer of the lower substrate 72 (which maybe considered a part of the thin film electronics layer 74) is patternedso that a plurality of the array element electrodes 38 (e.g. specificexamples of element electrodes are 38A and 38B in FIG. 5) are realized.These may be termed the array element electrodes 38. The term arrayelement electrode 38 may be taken in what follows to refer both to thephysical electrode structure 38 associated with a particular arrayelement, and also to the node of an electrical circuit directlyconnected to this physical structure. The reference electrode 28 isshown in FIG. 5 disposed upon the top substrate, but may alternativelybe disposed upon the lower substrate 72 to realize an in-plane referenceelectrode 28 geometry. The term reference electrode 28 may also be takenin what follows to refer to both or either of the physical electrodestructure and also to the node of an electrical circuit directlyconnected to this physical structure. The electro-wetting voltage may bedefined as the difference in voltage between the element electrode 38and the reference electrode 28.

FIG. 6A shows a circuit representation of the electrical load 40Abetween the element electrode 38 and the reference electrode 28 in thecase where a liquid droplet 4 is present. The liquid droplet 4 canusually be modelled as a resistor and capacitor in parallel. Typicallythe resistance of the droplet will be relatively low (e.g. if thedroplet contains ions) and the capacitance of the droplet will berelatively high (e.g. because the relative permittivity of polar liquidsis relatively high, e.g. ˜80 if the liquid droplet is aqueous). In manysituations the droplet resistance is relatively small and so, at thefrequencies of interest for electro-wetting, the liquid droplet 4 mayfunction in effect as an electrical short circuit. The hydrophobiccoatings 16 and 26 have electrical characteristics that may be modelledas capacitors, and the insulator 20 may also be modelled as a capacitor.The overall impedance between the element electrode 38 and the referenceelectrode 28 may be approximated by a capacitor whose value is typicallydominated by the contribution of the insulator 20 and hydrophobiccoatings 16 and 26 contributions, and which for typical layerthicknesses and materials may be of order a pico-Farad in value. Theoverall value of the electrical load 40A when a liquid droplet 4completely covers the element electrode 38 may be denoted as C_(I).

FIG. 6B shows a circuit representation of the electrical load 40Bbetween the element electrode 38 and the reference electrode 28 in thecase where no liquid droplet 4 is present. In this case the liquiddroplet 4 components are replaced by a capacitor representing thecapacitance of the non-polar fluid 34 which occupies the space betweenthe top and lower substrates. In this case the overall impedance betweenthe element electrode 38 and the reference electrode 28 may beapproximated by a capacitor whose value is dominated by the capacitanceof the non-polar fluid and which is typically small, of orderfemto-Farads. The overall value of the electrical load 40B when there isno liquid droplet 4 present at the element electrode 38 may be denotedas C_(OIL).

For the purposes of driving and sensing, the electrical load 40A/40Boverall functions in effect as a capacitor, whose value depends onwhether a liquid droplet 4 is present or not at a given elementelectrode 38. In the case where a droplet is present, the capacitance isrelatively high (typically of order pico-Farads) whereas if there is noliquid droplet 4 present the capacitance is low (typically of orderfemto-Farads). If a droplet partially covers a given electrode 38 thenthe capacitance may approximately represent the extent of coverage ofthe element electrode 38 by the liquid droplet 4.

FIG. 7 is a schematic diagram depicting an exemplary arrangement of thinfilm electronics 74 upon the lower substrate 72 in plan view. Eachelement of the electrode array 42 contains an array element circuit 84for controlling the electrode potential of a corresponding elementelectrode 38 and sensing the impedance present at the electrode 38.Integrated row driver circuit 76 and column driver circuit 78 are alsoimplemented in thin film electronics 74 to supply control signals to thearray element circuit 84.

A serial interface 80 may also be provided to process a serial inputdata stream and facilitate the programming of the required voltages tothe element electrodes 38 in the array 42. A voltage supply interface 83provides the corresponding supply voltages, top substrate drivevoltages, and other requisite voltage inputs as further describedherein. The number of connecting wires 82 between the lower substrate 72and external drive electronics, power supplies, and other components canbe made relatively few, even for large array sizes. Optionally theserial data input may be partially parallelized, for example if two datainput lines are used the first may supply data for columns 1 to X/2 andthe second for columns (1+X/2) to X with minor modifications to thecolumn driver circuit 78. In this way the rate at which data can beprogrammed to the array is increased, which is a standard technique usedin Liquid Crystal Display driving circuitry.

The thin film electronics also contains additionally sensor rowaddressing circuitry 88 for supplying control signals to the sensorcircuit inputs (e.g. RW) of the array element circuit 84, and columndetection circuits 86 for processing and reading out the output signalsfrom the sensor circuit part of the array element circuit 84.

Generally, an exemplary AM-EWOD device that includes thin filmelectronics 74 is configured as follows. The AM-EWOD device includes areference electrode 28 (which, optionally, could be an in-planereference electrode 28) and a plurality of array elements, each arrayelement including an array element electrode (e.g., array elementelectrodes 38).

Relatedly, the AM-EWOD device is configured to perform a method ofcontrolling an actuation voltage to be applied to a plurality of arrayelements. The AM-EWOD device includes reference electrode 28 and aplurality of array elements, each array element including an arrayelement electrode 38. The actuation voltage at each array element isdefined by a potential difference between the array element electrode 38and the reference electrode 28. The method of controlling the actuationvoltage includes the steps of supplying a voltage to at least a portionof the array element electrodes 38, and supplying a voltage signal tothe reference electrode 28.

Relatedly, the AM-EWOD device is further configured to perform a methodof sensing the impedance present at a plurality of array elements.Typically this may involve sensing the impedance at a plurality of arrayelement electrodes, with this impedance being a function of the number,size, position and constitution of one or more liquid droplets 4 presentwithin the array.

In general, therefore, an aspect of the invention is an active matrixelectro-wetting on dielectric (AM-EWOD) device. In exemplaryembodiments, the AM-EWOD device includes a plurality of array elementsarranged in an array of rows and columns, each of the array elementsincluding array element circuitry, an element electrode, and a referenceelectrode. The array element circuitry includes an actuation circuitconfigured to apply actuation voltages to the element and referenceelectrodes for actuating the array element, and an impedance sensorcircuit configured to sense impedance at the array element electrode todetermine a droplet property at the array element. The actuationcircuitry includes a memory part including a memory capacitor forstoring voltage data corresponding to either an actuated state or anunactuated state of the array element, and an input applied to thememory capacitor operates to effect an operation of the impedance sensorcircuit. In one circuit configuration, the input applied to the memorycapacitor operates to isolate the array element from the actuationvoltage during the operation of the impedance sensor circuit. In anothercircuit configuration, the memory circuit alternatively or additionallyoperates as a reference capacitor in the impedance sensor circuit.

Another aspect of the invention is a corresponding method of operatingan active matrix electro-wetting on dielectric (AM-EWOD) deviceincluding the steps of: arranging a plurality of array elements in anarray of rows and columns, each of the array elements including arrayelement circuitry, an element electrode, and a reference electrode, andthe array element circuitry comprises an actuation circuit and animpedance sensor circuit; applying actuation voltages with the actuationcircuit to the element and reference electrodes to actuate the arrayelement; wherein the actuation circuitry comprises a memory partincluding a memory capacitor, the method further comprising for storingvoltage data on the memory capacitor corresponding to either an actuatedstate or an unactuated state of the array element; applying an input tothe memory capacitor that operates to effect an operation of theimpedance sensor circuit; sensing impedance at the array elementelectrode with the impedance sensor circuit; and determining a dropletproperty at the array element based on the sensed impedance. The inputto the memory capacitor may effect an operation of the impedance sensorcircuit by isolating the array element from the actuation voltage duringthe operation of the impedance sensor circuit, and/or the input to thememory capacitor effects an operation of the impedance sensor circuit byoperating as a reference capacitor in the operation of the impedancesensor circuit.

FIG. 8 is a schematic diagram showing an example arrangement of thinfilm electronics 74 in the array element circuit 84. The array elementcircuit 84 may contain an actuation circuit 46, having inputs ENABLE,DATA and ACTUATE and an output which is connected to an elementelectrode 38. The array element circuit may further contain a sensorcircuit 48. The sensor circuit 48 has an input which is connected to anelement electrode, one or more row addressing lines RW and an outputwhich is connected to an output line OUT. Optionally certain circuitcomponents (e.g. transistors, capacitors) or addressing lines mayperform functions associated with the operation of both the actuationcircuit 46 and the sensor circuit 48, and these circuit components maybe considered to comprise a part of both the actuation circuit andsensor circuit.

FIG. 9 shows an array element circuit 84 according to a first embodimentof the invention. The array element circuit comprises n-type transistors52, 58 and 62, a p-type transistor 54 and capacitors 56, 44 and 60. Theelement electrode 38, the load present at the element electrode 40 andthe reference electrode 28 are shown since they play a role in theoperation of the array element circuit 84. The reference electrode maythus be considered to form a part of the array element circuit in thedescription that follows.

The array element circuit 84 may typically perform the functions of:

-   -   (i) Programming data to a memory element contained within the        actuator circuit and storing said data. The data to be        programmed is typically input by means of an addressing line        DATA which may be common to all elements within the same column        of the array. The programming of data may typically be        controlled by an addressing line ENABLE, which may typically be        common to all elements within the same row of the array.    -   (ii) Supplying a voltage signal to the array element electrode        38, for example as supplied by an input signal V1 which is        supplied to input ACTUATE, or alternatively switching the        element electrode 38 in to a high impedance state.

The array element circuit according to this embodiment and shown in FIG.9 is connected as follows. The drain of transistor 52 is connected tothe input DATA which may be common to all elements in the same column ofthe array. The gate of transistor 52 is connected to the input ENABLEwhich may be common to all elements in the same row of the array. Thesource of transistor 52 is connected to the gate of transistor 54.Capacitor 56, referred to more specifically as a memory capacitor, isconnected between the gate of transistor 54 and an addressing line RWSwhich may be common to all elements in the same row of the array. Thedrain of transistor 54 is connected to input signal ACTUATE which may becommon to all elements in the array. Capacitor 44, referred to morespecifically as the reference capacitor, is connected between theelement electrode 38 and addressing line RWS. Capacitor 60, referred tomore specifically as the sensor capacitor, is connected between theelement electrode 38 and the gate of sensing transistor 62. The drain ofsensing transistor 62 is connected to a DC voltage source VDD, which maybe common to all elements in the array. The source of sensing transistor62 is connected to output COL which may be common to all elements in thesame column of the array. Transistor 58 is connected between the gate ofsensing transistor 62 and a voltage supply VRST which may be common toall elements in the array. The gate of transistor 58 is connected to aninput signal RST which may be common to all elements in the same row ofthe array. The actuator circuit 46 comprises transistor 52, transistor54 and memory capacitor 56 and the terminal connections DATA, ENABLE andACTUATE. The sensor circuit comprises memory capacitor 56, referencecapacitor 44, sensor capacitor 60, transistor 54, transistor 58,transistor 60, addressing lines RST, VRST and output line COL.Transistor 54, capacitor 56, the element electrode 38, the electrodeload 40, the reference electrode 28 and addressing line RWS form a partof both the actuator circuit and sensor circuit.

The operation of the array element circuit 84 is described as follows.The array element circuit performs two functions including (1) actuationand (2) impedance sensing.

(1) Actuation

The actuation circuit has two parts, a memory part and an actuationpart. Generally, the memory part is configured for storing data, whichmay be digital data, corresponding to either an actuated state or anunactuated state of the array element, and the actuation part isconfigured for supplying the actuation voltages to the element electrodeand the reference electrode.

The memory part is explained as follows. The memory part includes twotransistors 52 and 54 for controlling the storing of the voltage data onthe memory capacitor. Transistor 52 and memory capacitor 56 between themfunction as a Dynamic RAM (DRAM) memory element, capable of programmingand storing data within the array element circuit 84. To program data, avoltage is programmed onto the column addressing line DATA. The ENABLEline is then taken high to switch transistor 52 on. The voltage on DATAis then programmed onto capacitor 56 and is held there once ENABLE istaken low, irrespective of how the voltage on input line DATA maysubsequently be varied after ENABLE is taken low. In typical operation,the programmed voltage may be digital and be approximately 0.5×V_(EW)(for programming a “0” state, the droplet being unactuated in thisstate) or −0.5×V_(EW) Volts (for programming a “1” state, the dropletbeing actuated in this state).

The actuation part is explained as follows. An AC voltage signal V1 isapplied to input ACTUATE and an AC voltage signal V2 is applied to thereference electrode 28. V1 and V2 are arranged to be in anti-phase (e.g.180 degrees out of phase), or substantially in antiphase (e.g. a highphase angle out of phase for example greater than 90 degreed out ofphase, or greater than 135 degrees out of phase or greater than 160degrees out of phase). An example arrangement of voltage signals V1 andV2 is shown in FIG. 10. Each of V1 and V2 are switched between a lowlevel of −0.5×V_(EW) Volts and a high level of 0.5×V_(EW), V1 is highwhen V2 is low and vice versa. The element electrode 38 is actuated whena “1” is programmed to the memory (a voltage of −0.5×V_(EW) programmedto the gate of transistor 54). In this case transistor 54 is turned onand so voltage signal V1 is transmitted to the element electrode 38. Thevoltage developed across the electrical load 40 (the electro-wettingvoltage) is therefore V1−V2 which is an AC voltage waveform that variesin time between −V_(EW) and +V_(EW).

The element electrode 38 is non-actuated when a “0” is programmed to thememory (a voltage of 0.5×V_(EW) programmed to the gate of transistor54). In this case transistor 54 is turned off. The element electrode 38therefore exists in a high impedance state. There are two differentcases where (1) a droplet is present at the element electrode 38 (theelectrical load 40A is as FIG. 6A) and (2) no droplet is present at theelement electrode 38 (the electrical load 40B is as FIG. 6B).

Case 1—Droplet Present:

Where a droplet is present, the dominant electrical coupling of theelement electrode 38 is to the reference electrode 28 via the electricalload 40. As previously explained, the electrical load in this case 40Amay be approximated by a capacitor whose value is typically of order apico-Farad. The capacitance of the electrical load 40A will thendominate over other parasitic impedances in the circuit (e.g. thatassociated with the source-gate capacitance of transistor 54, typicallyof order femto-Farads). The electrical potential of the elementelectrode 38 will therefore track the potential of the referenceelectrode 28, and will thus correspond to a good approximation to thevoltage signal V2. This being the case, the potential developed betweenthe element electrode 38 and the reference electrode 28 willapproximately be zero. The liquid droplet 4 will therefore be in anon-actuated state, the contact of the liquid droplet 4 with thehydrophobic coating 16 will not be energized and the liquid droplet 4will not experience an electro-wetting force.

Case 2—No Droplet Present:

When no liquid droplet 4 is present, the capacitance between the elementelectrode 38 and the reference electrode 28 is very small as previouslyexplained. The element electrode 38 is therefore now in a high impedancestate and its effective potential is only poorly defined, beingdependent on the multiple small parasitic capacitances and resistanceswithin the circuit (e.g. the small electrical load 40B capacitance tothe reference electrode 28, the small parasitic source to gatecapacitance of transistor 54, and the large off resistance of transistor54). It may therefore be unclear what the effective potential of theelement electrode 38 is and therefore the extent to which the elementelectrode 38 remains effectively non-actuated.

However, the situation is such that, even with the potential of theelement electrode 38 being poorly defined in Case 2, the device canstill support the correct transport of liquid droplets 4. This isbecause if any liquid droplet does encroach into the position of thenon-actuated element electrode 38, there is associated with this asignificant increase in the capacitance between the reference electrode28 and element electrode 38B. In this situation, the potential of theelement electrode 38B becomes approximately that of the referenceelectrode 28 by means of the capacitive coupling through the liquiddroplet 4. In other words the situation begins to resemble more closelyCase 1 than Case 2, and the element electrode is in a non-actuatedstate. This effect is explained in further detail in co-pendingapplication UK Application GB1500261.1.

An advantage of the array element actuation circuit and method ofdriving described in this embodiment is that the electro-wetting voltagein the actuated state is switched between +V_(EW) and −V_(EW).Therefore, AC electro-wetting is implemented. This is achieved whilstonly requiring the array element circuit 84 to switch approximatelyV_(EW) between the terminals of any transistor in the circuit (for thereasons why this may only be approximate see the more detaileddescription in UK Application GB1500261.1). This is an importantadvantage of the invention, since typically electro-wetting requiresrelatively high voltages to actuate the liquid droplets, whilst typicalelectronics technologies for realizing the thin film electronics 74impose limitations on the maximum voltage applied to the transistors(e.g. due to reliability concerns).

A further advantage of this embodiment is that the actuation part of thearray element circuit 84 has been implemented with only two transistorsand one capacitor. Smaller array elements/element electrodes maytherefore be realized. Smaller array elements may be advantageous for atleast three reasons. Firstly, smaller liquid droplets may bemanipulated. Secondly, if using larger liquid droplets, sub-dropletresolution of actuation may be achieved. Thirdly, smaller array elementsizes facilitate the design and fabrication of a very large format arraywhich may have a total number of array elements in excess of 1 millionand which may be able to manipulate tens to hundreds of thousands ofdroplets simultaneously and independently. These advantages aredescribed in more detail in UK Application GB1500261.1, incorporated byreference.

(2) Impedance Sensor

The operation of the impedance sensor function is based on theprinciples of U.S. Pat. No. 8,547,111 (Hadwen, issued Oct. 1, 2013) andU.S. Pat. No. 8,653,832 (referenced in the background section),incorporated by reference.

The circuit works in essence by comparing the impedance of the referencecapacitor, capacitor 44, with the electrical load 40 presented at theelement electrode 38. The operation of the impedance sensor function isexplained with reference to the exemplary timing diagram shown in FIG.11, showing the timings of the addressing signals RST, RWS and ACTUATEduring the sensor operation as follows:

-   -   The ACTUATE signal is first taken to a low level.    -   The reset signal RST is taken high for a period of time which        may be denoted the reset period. During the reset period        transistor 58 is turned on and the gate of sensing transistor 62        is charged to the reset potential VRST. Typically the value of        VRST may be chosen such that sensing transistor 62 is turned        off. Following the conclusion of the reset period, RST is taken        low so that transistor 58 remains turned off. The potential at        the gate of transistor 62 remains at substantially VRST with        this node now in a high impedance state.    -   A row select line that provides a row select signal input RWS is        now taken high. This has two effects on the circuit as follows:        -   1) The row select line provides an input to the memory            capacitor 56, and the input from the row select line to the            memory capacitor 56 operates to isolate the array element            from the actuation voltage during the operation of the            impedance sensor circuit. Taking RWS high causes a            perturbation in the potential at the gate of transistor 54.            Charge is injected across capacitor 56 with the result that            the potential at the gate of transistor 54 is increased. The            magnitude of the RWS pulse amplitude (ΔVRWS) may be chosen            such that perturbation in the potential of the gate of            transistor 54 is sufficiently large such that in the case            where a “0” is programmed to the memory of the array element            (such that prior to the perturbation transistor 54 was            turned on), the perturbation results in transistor 54 now            being turned off for the duration of the RWS pulse. The            effect of the RWS pulse on this part of the circuit is            therefore to ensure that transistor 54 is turned off. It            therefore isolates the ACTUATE signal from the element            electrode for the duration of the RWS pulse, thus            facilitating the sensing operation.        -   2) The impedance sensor circuit operates by forming a            potential divider between the reference capacitor and the            reference capacitor 44. Specifically, the row select signal,            RWS, is also connected to one of the terminals of reference            capacitor 44. A further result of the row select signal is            the injection of charge across capacitor 44 thus perturbing            the potential at the element electrode 38. Capacitor 44,            whose value may be represented by C_(S), forms a potential            divider with the load circuit 40 and the sensor capacitor            60, the latter whose value may be represented as C_(C).            Therefore, in the case where a liquid droplet 4 is presented            at the element electrode 38, the perturbation of the            potential at the element electrode is approximately given            by:

$\begin{matrix}{{\Delta\; V_{A}} = {V_{0} + {\Delta\;{VRWS}\frac{C_{S}}{C_{S} + C_{I} + C_{C}}}}} & \left\lbrack {{equation}\mspace{14mu} 1a} \right\rbrack\end{matrix}$

-   -   -   Where V₀ is the initial potential at the element electrode            and C_(I) is as previously defined.        -   In the case where no liquid droplet 4 is present at the            element electrode 38 the perturbation at the element            electrode is given by:

$\begin{matrix}{{\Delta\; V_{A}} = {V_{0} + {\Delta\;{VRWS}\frac{C_{S}}{C_{S} + C_{OIL} + C_{C}}}}} & \left\lbrack {{equation}\mspace{14mu} 1b} \right\rbrack\end{matrix}$

-   -   -   Where C_(OIL) is as previously defined.        -   The perturbation of the potential at the element electrode            further causes an injection of charge across the sensor            capacitor 60. The potential at the gate of sensing            transistor 62 therefore becomes:

$\begin{matrix}{{\Delta\; V_{B}} = {{VRST} + {\Delta\; V_{A}\frac{C_{C}}{C_{par} + C_{C}}}}} & \left\lbrack {{equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

-   -   -   Where C_(par) represents the parasitic capacitances at the            gate of sensing transistor 62 and associated with            transistors 58 and 62.            The overall function of the RWS pulse input from the row            select line to the reference capacitor 40 is therefore            twofold:

-   (1) To electrically isolate the element electrode from the input    line ACTUATE by turning off transistor 54 (in the case where it was    not turned off already).

-   (2) To perturb the potential at the gate of sensing transistor 62 by    an amount that depends on the impedance of the load circuit 40. In    the case where a droplet is present at the element electrode 38, the    perturbation (ΔV_(B)) is small and sensing transistor 62 may remain    turned off, or only slightly turned on for the duration of the RWS    pulse, and a small current is sourced through the output COL. In the    case where no droplet is present at the element electrode 38, the    perturbation (ΔV_(B)) is large and sensing transistor 62 is turned    on so that a large current is sourced through the output COL. The    current through the output COL may typically be measured by means of    standard circuitry in the column detection circuit 86. This may be    done using standard techniques for CMOS image sensor as is very well    known.

It will be noted from the description above that an important feature ofthe circuit is that the gate of sensing transistor 62 is AC coupled tothe element electrode. An advantage of this arrangement is that thepotential at the gate of sensing transistor 62 V_(B) (as produced by theperturbing effect of the RWS pulse and the subsequent potentialdividing) is independent of the initial voltage V₀ of the elementelectrode 38. A further advantage is that the total range of voltagethat may be produced at the gate of sensing transistor 62 may be muchsmaller than the range of voltages that may be produced at the elementelectrode 38. Therefore transistors 62 and 58 may be formed from astandard low voltage device construction (e.g. 5 Volt or 8 Volttransistors).

It will be appreciated in the above description that a critical part ofthe sensor operation is that transistor 54 is turned off, thus isolatingthe element electrode from the signal ACTUATE for the duration of theRWS pulse. If this was not the case, the potential of the elementelectrode would remain substantially pinned to the potential of ACTUATEand the potential divider would not work as described. Accordingly, theinventors have realized that the disconnection of the actuate signalfrom the element electrode may be achieved without recourse to anadditional isolation transistor (for example as shown in the prior artcircuit FIG. 3 disclosed in UK Application GB1500261.1. In this priorart the disconnecting of the voltage signal applied to ACTUATE from theelement electrode 38 is implemented by means of an additional switchtransistor and an additional addressing signal SEN (connected to thegate of said additional transistor).

However, in the array element circuit of this embodiment of the presentinvention, transistor 54 is arranged to fulfill an additional function(compared to the prior art circuit of FIG. 3) in effecting the isolationof ACTUATE from the element electrode 38 during the sense operation.Transistor 54 fulfills this role in addition to its function as anaccess transistor during the actuate operation. Transistor 54 thusperforms a dual function and plays a role in both the actuate and senseoperations. Furthermore, the array element circuit of this embodimentalso does not require an additional addressing line (for example asaddressing line SEN in the prior art circuit of FIG. 3). The addressingline RWS is configured to perform the dual operations of both turningoff transistor 54 and providing an interrogation signal to the potentialdivider part of the circuit.

A significant advantage of this embodiment is that the array elementcircuit 84, having both an actuate and sensor function, has beenimplemented with a minimal number of circuit components and addressinglines. For example, the array element circuit has one fewer transistorand one fewer addressing line than the array element circuit in priorart of UK Application GB1500261.1 and as reproduced in FIG. 3.

Reducing the complexity and the number of transistors in the arrayelement circuit 84 is advantageous for several reasons:

-   -   Smaller array elements/element electrodes may be realized.        Typically it is often the case that the minimum achievable array        element size is set by the limitations of the thin film        electronics and the design for fabrication requirements (design        rules) dictating the layout of the array element circuit 84 in        thin film electronics. A simpler circuit (fewer transistors)        therefore enables smaller array elements to be designed and        fabricated. Smaller array elements may be advantageous for at        least three reasons. Firstly, smaller liquid droplets may be        manipulated. This is particularly important for applications        involving the manipulation or analysis of single cells or single        molecules. Secondly, if using larger liquid droplets,        sub-droplet resolution of actuation may be achieved. This may        improve the capabilities of the device, for example enabling        more accurate splitting or faster mixing. Thirdly, smaller array        element sizes facilitate the design and fabrication of a very        large format array which may have a total number of array        elements in excess of 1 million and which may be able to        manipulate tens to hundreds of thousands of droplets        simultaneously and independently.    -   A smaller and simpler design of array element circuit 84 may        facilitate increased manufacturing yield and hence lower cost of        the device.    -   A smaller and simpler design of array element circuit 84 may        facilitate increased optical transparency of the device. This        may be important, for example, if the device is being used to        implement chemical or biochemical tests that result in a change        in the optical properties (e.g. fluorescence, absorbance) of one        or more liquid droplets and that by measurement of this change        in optical property the device may be read out.    -   A smaller and simpler design of array element circuit 84 may        free up space within the array element to implement other        electronic functions into the array element, e.g. temperature        sensing, bio-sensing and like operations.

Typically, but not necessarily, on the rows of the sensor not beingsensed, RST is maintained high such that the potential at the gate ofsensing transistor 62 is pinned to VRST so that sensing transistor 62 ismaintained switched off for all elements in the rows not being sensed.

Optionally, the signal ACTUATE may be common only to elements in thesame row of the array. If this is the case it is not necessary tomaintain ACTUATE at a low level for the rows of the array not beingsensed, providing that perturbations of the potential at the gate ofsensing transistor 62 are not sufficiently large to turn sensingtransistor 62 on or partially on in any of the rows of the array notbeing sensed. This condition may be achieved by maintaining RST high onthe rows not being sensed for the duration of the sensing operation.Making the ACTUATE signal common only to elements in the same row of thearray has the advantage that it is not necessary to interrupt theactuation operation on the rows of the array not being sensed.

Alternatively, making the ACTUATE signal common to all elements in thearray has the advantages that in the layout of the circuit a singlephysical line may supply the signal to two rows of the array, thusminimizing the area required and the reducing the minimum size of arrayelement that may be realized within the design rules of themanufacturing process.

According to a further variant, it is also possible to re-start theACTUATE signal following the rising edge of the RWS pulse on all rows ofthe array. For the rows of the array not being sensed, this will resultin the ACTUATE signal coupling to the element electrode, or not,according whether a “1” or a “0” has been programmed to the arrayelement. For the row being sensed, however, the perturbation at the gateof transistor 54 ensures that for elements in the row being sensed thetransistor 54 always remains turned off. Therefore, for this row of thearray the ACTUATE signal has no effect and the sensing operation canfunction as described.

An AM-EWOD device according to a second embodiment of the invention iscomparable to the first embodiment except that an alternative design ofarray element circuit is employed as shown in FIG. 12. The topology ofthe array element circuit is as the first embodiment with the exceptionthat transistor 54 is an n-type device and the gate of transistor 54 isconnected to another row select line that provides a row addressingsignal input RWSB which may be common to all elements in the same row ofthe array. The operation of the array element circuit according to thisembodiment is comparable as described for the first embodiment with thefollowing differences:

-   -   Since transistor 54 is n-type, it is turned on when the        potential at its gate is high, and turned off when the potential        at its gate is low. To program the memory for control of the        actuation voltage, a high voltage is written to the gate of the        transistor 54 to correspond to programming “1” and a low voltage        is written to the gate of transistor 54 to correspond to        programming a “0”.    -   RWSB from the another row select line may constitute a row        addressing signal that is the logical inverse input of the RWS        signal provided by the first row select line. Therefore, the        timing signals of the second embodiment may be as shown in        FIG. 13. The effect of the RWSB signal is that on falling edge,        charge is injected across memory capacitor 56 and the potential        at the gate of transistor 54 also falls correspondingly. The        voltage swing associated with the RWSB transition and the        operating voltages of the circuit may be arranged such that the        following the RWSB falling edge transistor 54 is guaranteed to        be always turned off, regardless of whether a “1” or a “0” was        written to the memory.

The array element circuit of the second embodiment thus operates in avery similar way to that previously described for the first embodiment.As before, transistor 54 has a dual function, as an access transistorduring the actuation operation and as an isolation transistor during thesensing operation.

Compared to the first embodiment, the array element of the secondembodiment has one additional row addressing line RWSB. However, thesecond embodiment has the additional advantage that it only requiresn-type transistors to implement the array element circuit. This maytherefore facilitate fabrication of the AM-EWOD device with a simplerand lower cost fabrication process. This embodiment may also thereforebe particularly suitable for AM-EWOD devices fabricated using a singlechannel thin film transistor fabrication process, for example based onamorphous silicon TFTs or oxide TFTs (e.g. zinc oxide orindium-gallium-zinc-oxide TFTs).

Optionally, the timing of the falling edge of the voltage signalsupplied to RWSB may be delayed slightly with respect to the timing ofthe RWS rising edge signal. This may be advantageous to ensure thattransistor 54 is fully turned off before RWS transitions high andpotential division occurs. Operating in this way may thus prevent a racecondition between transistor 54 turning off and potential divisiontaking place leading to improved performance and reduced noise on thesensor signal output.

An AM-EWOD device according to a third embodiment of the invention iscomparable the first embodiment with an alternative design of arrayelement circuit as shown in FIG. 14. This array element circuit iscomparable to the array element circuit of the first embodiment with thedifference a common addressing line is connected to each of the twotransistors 58 and 62 for setting the sensing voltage at the sensorcapacitor. In particular, the drain of transistor 58 and the drain ofsensing transistor 62 are connected to a common addressing line thatprovides input signal RSTL, which may be common to all elements in thesame row of the array. The operation of the array element circuitaccording to this embodiment is similar to as previously described forthe first embodiment, with the exception that the functions of the DCsupply voltages VDD and VRST are combined into a single supply VRSTLwhich may be switched between the voltage levels VRST and VDD during theoperation of the circuit.

This embodiment exploits the fact that both of the following DCpotential levels of nodes within the circuit are unimportant for theoperation of the sensor function:

-   -   The potential at the drain of sensing transistor 62 during the        reset operation. This is because sensing transistor 62 remains        switched off during the reset operation. The current through        sensing transistor 62 is small and almost independent of the        potential of the drain of sensing transistor 62 during the reset        operation and has no influence on the overall operation of the        circuit.    -   The potential at the drain of transistor 58 during the time when        RWS is taken high. This is because the transistor 58 remains        switched off once the reset operation is concluded. The        potential at the drain of transistor 58 therefore has no        influence on the potential at the gate of sensing transistor 62        once the reset operation is concluded.

An example timing chart showing the operation of the sensor function ofthe array element circuit according to this embodiment is shown in FIG.15. The addressing line RWSL is modulated between voltage levels VRSTand VDD. Initially VRSTL may be at VDD, then VRSTL is taken to voltagelevel VRST shortly before the rising edge of RST. Following theconclusion of the reset period and the falling edge of RST, RSTL istaken back to voltage VDD in time for the rising edge of RWS. Otherwisethe device and array element circuit functions comparably as previouslydescribed.

An advantage of this embodiment is that by combining the voltagesupplies VDD and VRST into a single addressing line RSTL, the arrayelement circuit requires one fewer addressing line than the firstembodiment. It therefore may be made smaller, with all the advantages ofa smaller circuit as previously described.

A fourth embodiment of the invention is comparable to the firstembodiment with an alternative design of array element circuit as shownin FIG. 16.

The connectivity of the array element circuit is as follows:

The drain of transistor 52 is connected to the input DATA which may becommon to all elements in the same column of the array. The gate oftransistor 52 is connected to the input ENABLE which may be common toall elements in the same row of the array. The source of transistor 52is connected to the gate of transistor 54. Capacitor 56 is connectedbetween the gate of transistor 54 and an addressing line RWS which maybe common to all elements in the same row of the array. The drain oftransistor 54 is connected to input signal ACTUATE which may be commonto all elements in the array. Transistor 64, which is n-type, isconnected between the source of transistor 54 and the element electrode38. Transistor 66, which is p-type is connected between the gate oftransistor 54 and the element electrode 38. The gate of transistor 64 isconnected to the gate of transistor 66 and sensor input addressing lineSEN for actuating the impedance sensor circuit, which may be common toall elements in the same row of the array. Capacitor 60 is connectedbetween the element electrode 38 and the gate of sensing transistor 62.The drain of sensing transistor 62 is connected to a DC voltage sourceVDD, which may be common to all elements in the array. The source ofsensing transistor 62 is connected to output COL which may be common toall elements in the same column of the array. Transistor 58 is connectedbetween the gate of sensing transistor 62 and a voltage supply VRSTwhich may be common to all elements in the array. The gate of transistor58 is connected to an input signal RST which may be common to allelements in the same row of the array. The actuator circuit 46 comprisesof transistor 52, transistor 54, capacitor 56 and transistor 64 and theinputs DATA, ENABLE and ACTUATE. The sensor circuit comprises ofcapacitor 56, capacitor 60, transistor 66, transistor 58, transistor 62,and terminal connections RWS, RST, SEN, VRST and COL. Transistors 64 and66, capacitor 56, the element electrode 38, the electrode load 40 andthe reference electrode 28 form a part of both the actuator circuit andsensor circuit.

The operation of the array element circuit is described as follows:

(1) Actuation

The actuation function operation is similar to as previously describedfor the second embodiment of the invention. During actuation theaddressing line SEN is at a high voltage level. Therefore transistor 64is turned on and transistor 66 is turned off. In the case where a “1” isprogrammed to the array element, the gate of transistor 54 is at a highvoltage level, transistor 54 is turned off and the input signal ACTUATEis connected through to the element electrode 38. In the case where a“0” is programmed to the array element, the gate of transistor 54 is ata low voltage level, transistor 54 is turned off and the elementelectrode is in a high impedance state. In other respects the ACTUATIONoperation functions as previously described.

(2) Impedance Sensor

The impedance sensor functions in a similar manner as previouslydescribed, with the difference that memory capacitor 56 also functionsas the reference capacitor to which the load impedance 40 is comparedwithin the operation of the potential divider. The separate referencecapacitor 44 of previous embodiments therefore is eliminated. Theoperation of the impedance sensor function of the array element circuitof FIG. 16 is described with reference to the timing diagram shown inFIG. 17.

At the beginning of the sensing operation, the input signal ACTUATE istaken to a low level. The input signal SEN is then taken to a low level.This has the effect of turning off transistor 64, and thus isolating thesource of transistor 54 from the element electrode. Taking SEN to a lowlevel also turns on transistor 66 so that the gate of transistor 54becomes connected to the element electrode 38. The RST signal line isthen taken high thus resetting the potential at the gate of sensingtransistor 62 to the reset voltage level VRST as previously described.RST is then taken low again, followed by RWS being taken high. Apotential divider is thus formed between memory capacitor 56, the loadimpedance 40 and sensor capacitor 60. The value of the voltageperturbation at the element electrode is described by equation 1a or 1b,as previously presented and explained, for the cases where a liquiddroplet 4 is present or absent at the element electrode respectively,and where C_(S) is the capacitance of capacitor 56. In other respectsthe operation of the impedance sensor is as previously described.

Alternatively and optionally, ACTUATE may be taken to a high level ormay be alternated between a high and low level for the duration of thesensing operation. The operation of the impedance sensor functionaccording to this embodiment of the invention utilizes memory capacitor56 as part of both the actuate and the sensing operation. In theoperation of the actuation function, capacitor 56 forms part of thememory function, acting in effect as the storage capacitor in a DRAMcircuit. The array element is programmed by writing and storing avoltage, the programmed voltage being stored on capacitor 56, andcontrolling the potential at the gate of transistor 54. In the operationof the impedance sensor, one terminal of the capacitor 56 becomesconnected to the element electrode (for the time for which transistor 66is switched on) and the pulsing of the RWS addressing line injectscharge across the capacitor 56 and onto the element electrode. Capacitor56 thus functions as the reference capacitor to which the load impedance40 is compared in the operation of the potential divider. Capacitor 56therefore has a dual function, playing a role in the operation of boththe actuate and impedance sensing operations.

An advantage of the fourth embodiment is that it removes the need for acapacitor connected directly between the element electrode 38 and theRWS addressing line (e.g. reference capacitor 44 as shown in FIG. 9).This capacitor, typically of value around 1 pF, is physically quitelarge and occupies a substantial portion of the total layout area of theelement electrode circuit. The removal of this capacitor may thereforesubstantially reduce the physical size of the array element circuit,with all the advantages as previously described.

Variants of this embodiment combining features of previous embodimentsare also possible. For example, a single channel circuit architecture(including only n-type transistors) could be realized by makingtransistor 66 of n-type construction and connecting the gate oftransistor 66 to an additional row addressing line SENB which is drivenby the logical inverse of the supply SEN.

A further variant is that the DC voltage supplies VRST and VDD may becombined, as for example described with respect to the third embodimentof the invention.

An AM-EWOD device according to a fifth embodiment of the invention iscomparable to the fourth embodiment with a modified array elementcircuit as shown in FIG. 18. The array element circuit is modified fromthat of the fourth embodiment in that the gate of transistor 58 isconnected to the voltage signal ACTUATE and the input signal RST is nolonger required. According to the operation of this embodiment, thetimings of the ACTUATE signal are varied to those shown in FIG. 19.According to this embodiment the signal ACTUATE performs the dualfunction of droplet actuation and the re-setting of the potential at thegate of sensing transistor 62.

An advantage of the fifth embodiment is that in comparison to the fourthembodiment one fewer addressing line is required. This affords the sameadvantages as previously described of reduced array element size.

An AM-EWOD device according to a sixth embodiment of the invention hasan array element circuit as shown in FIG. 20. This embodiment iscomparable to the fourth embodiment but additionally incorporates theconcept of the third embodiment of combining the DC voltage suppliedVRST and VDD into a single supply line RSTL whose voltage may bemodulated during the operation of the sense function. The timing diagramfor the operation of the array element circuit of FIG. 20 is as shown inFIG. 21, and is comparable to the timing diagram of FIG. 17 with theaddition that timings are applied to the voltage signal RSTL.Accordingly, RSTL may assume a voltage of VRST during the performing ofthe reset operation (i.e. when RST) is taken high, re-setting thevoltage at the gate of sensing transistor 62 to VRST. Likewise RSTL mayassume a voltage of VDD during the operation of the row select function(RWS high).

The sixth embodiment has all of the advantages of the fourth embodimentwith the additional advantage that the array element circuit requiresone fewer addressing lines in comparison to the array element circuit ofthe fourth embodiment. This may enable a smaller array element size withthe same advantages as previously described.

Whilst in the preceding embodiments, the invention has been described interms of an AM-EWOD device utilizing thin film electronics 74 toimplement array element circuits and driver systems in thin filmtransistor (TFT) technology, the invention could equally be realizedusing other standard electronic manufacturing processes, e.g.Complementary Metal Oxide Semiconductor (CMOS), bipolar junctiontransistors (BJTs), and other suitable processes.

An aspect of the invention, therefore, is an active matrixelectro-wetting on dielectric (AM-EWOD) device. In exemplaryembodiments, the AM-EWOD device includes a plurality of array elementsarranged in an array of rows and columns, each of the array elementsincluding array element circuitry, an element electrode, and a referenceelectrode. The array element circuitry comprises an actuation circuitconfigured to apply actuation voltages to the element and referenceelectrodes for actuating the array element, and an impedance sensorcircuit configured to sense impedance at the array element electrode todetermine a droplet property at the array element. The actuationcircuitry comprises a memory part including a memory capacitor forstoring voltage data corresponding to either an actuated state or anunactuated state of the array element, and an input applied to thememory capacitor effects an operation of the impedance sensor circuit.The AM-EWOD device may include one or more of the following features,either individually or in combination.

In an exemplary embodiment of the AM-EWOD device, the AM-EWOD devicefurther includes a row select line that is configured to provide aninput to the memory capacitor, and the input from the row select line tothe memory capacitor effects an operation of the impedance sensorcircuit by operating so as to isolate the array element from theactuation voltage during the operation of the impedance sensor circuit.

In an exemplary embodiment of the AM-EWOD device, the AM-EWOD devicefurther includes a transistor connected between an actuation signal andthe element electrode and whose gate is connected to the memorycapacitor, wherein the input from the row select line to the memorycapacitor operates to turn off the transistor to isolate the arrayelement from the actuation voltage during the operation of the impedancesensor circuit.

In an exemplary embodiment of the AM-EWOD device, the sensor circuitcomprises a reference capacitor and a sensor capacitor, and theimpedance sensor circuit is configured to operate by forming a potentialdivider circuit comprising the reference capacitor and the sensorcapacitor.

In an exemplary embodiment of the AM-EWOD device, the memory capacitoracts as the reference capacitor.

In an exemplary embodiment of the AM-EWOD device, the AM-EWOD devicefurther includes a row select line that provides an input to thereference capacitor, wherein an input from the row select line to thereference capacitor effects an operation of the impedance sensor circuitby perturbing a potential at the array element as part of operating theimpedance sensor circuit, and the droplet property is determined basedon an amount of change of the potential at the array element.

In an exemplary embodiment of the AM-EWOD device, the transistorconnected between the memory capacitor and the impedance sensor circuitis a p-type transistor.

In an exemplary embodiment of the AM-EWOD device, the transistorconnected between the memory capacitor and the impedance sensor circuitis an n-type transistor, and further comprising another row select linethat provides an input to the n-type transistor. The row select line isconnected to the reference capacitor, an input from the row select lineto the reference capacitor perturbs the potential at the array elementas part of operating the impedance sensor circuit, and an input from theanother row select line to the n-type transistor is an inverse input ofthe input from the row select line to the reference capacitor.

In an exemplary embodiment of the AM-EWOD device, the impedance sensorcircuit further comprises a capacitor connected between the elementelectrode and a gate of a sensing transistor, and the impedance at thearray element is sensed based on a change in a sensing voltage coupledacross the capacitor.

In an exemplary embodiment of the AM-EWOD device, the AM-EWOD devicefurther includes a common addressing line that is connected to each ofthe two transistors for setting the sensing voltage at the gate of thesensing transistor and providing a bias supply to the drain of thesensing transistor.

In an exemplary embodiment of the AM-EWOD device, the AM-EWOD devicefurther includes a sensor input line for actuating the impedance sensorcircuit, and the memory capacitor operates as the reference capacitor inthe operation of the impedance sensor circuit.

In an exemplary embodiment of the AM-EWOD device, the AM-EWOD devicefurther includes a sensor input line for actuating the impedance sensorcircuit. The memory capacitor operates as the reference capacitor in theoperation of the impedance sensor circuit, and the actuating voltage foractuating the array element also is inputted to the one of thetransistors of the impedance sensor circuit for resetting the sensingvoltage.

In an exemplary embodiment of the AM-EWOD device, the actuator circuitfurther comprises two transistors for controlling the storing of thevoltage data on the memory capacitor.

Another aspect of the invention is a method of operating an activematrix electro-wetting on dielectric (AM-EWOD) device. In exemplaryembodiments, the operating method includes the steps of: arranging aplurality of array elements in an array of rows and columns, each of thearray elements including array element circuitry, an element electrode,and a reference electrode, and the array element circuitry comprises anactuation circuit and an impedance sensor circuit; applying actuationvoltages with the actuation circuit to the element and referenceelectrodes to actuate the array element; wherein the actuation circuitrycomprises a memory part including a memory capacitor, the method furthercomprising for storing voltage data on the memory capacitorcorresponding to either an actuated state or an unactuated state of thearray element; applying an input to the memory capacitor that operatesto effect an operation of the impedance sensor circuit; sensingimpedance at the array element electrode with the impedance sensorcircuit; and determining a droplet property at the array element basedon the sensed impedance. The operating method may include one or more ofthe following features, either individually or in combination.

In an exemplary embodiment of the operating method, the AM-EWOD devicefurther comprises a row select line that provides an input to the memorycapacitor, the operating method further comprising providing the inputfrom the row select line to the memory capacitor to effect an operationof the impedance sensor circuit by isolating the array element from theactuation voltage during the operation of the impedance sensor circuit.

In an exemplary embodiment of the operating method, the AM-EWOD devicefurther comprises a transistor connected between an actuation signal andthe element electrode and whose gate is connected to the memorycapacitor, wherein the input from the first row select line to thememory capacitor operates to turn off the transistor to isolate thearray element from the actuation voltage during the operation of theimpedance sensor circuit.

In an exemplary embodiment of the operating method, the impedance sensorcircuit comprises a reference capacitor and a sensor capacitor, andsensing the impedance at the array element electrode comprises forming apotential divider comprising the reference capacitor and the sensorcapacitor.

In an exemplary embodiment of the operating method, the memory capacitoreffects an operation of the impedance sensor circuit by operating as thereference capacitor in the operation of the impedance sensor circuit.

In an exemplary embodiment of the operating method, the row select lineis connected to the reference capacitor, and sensing impedance at thearray element electrode comprises: perturbing the potential at the arrayelement with an input from the row select line to the referencecapacitor; sensing an amount of perturbing of the potential at the arrayelement electrode based on a change of impedance at the array elementelectrode; and determining a droplet property at the array element basedon an amount of perturbing of the potential at the array elementelectrode.

In an exemplary embodiment of the operating method, the AM-EWOD devicefurther comprises a sensor input line, the operating method furthercomprising actuating the impedance sensor circuit with an input to thesensor input line.

Although the invention has been shown and described with respect to acertain embodiment or embodiments, equivalent alterations andmodifications may occur to others skilled in the art upon the readingand understanding of this specification and the annexed drawings. Inparticular regard to the various functions performed by the abovedescribed elements (components, assemblies, devices, compositions,etc.), the terms (including a reference to a “means”) used to describesuch elements are intended to correspond, unless otherwise indicated, toany element which performs the specified function of the describedelement (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein exemplary embodiment or embodiments of theinvention. In addition, while a particular feature of the invention mayhave been described above with respect to only one or more of severalembodiments, such feature may be combined with one or more otherfeatures of the other embodiments, as may be desired and advantageousfor any given or particular application.

Optionally the device may also be arranged such that embodiments of theinvention may be utilized in just a part or sub-array of the entiredevice. Optionally some or all of the multiple different embodiments maybe utilized in different rows columns or regions of the device.

INDUSTRIAL APPLICABILITY

The described embodiments could be used to provide an enhance AM-EWODdevice. The AM-EWOD device could form a part of a lab-on-a-chip system.Such devices could be used in manipulating, reacting and sensingchemical, biochemical or physiological materials. Applications includehealthcare diagnostic testing, material testing, chemical or biochemicalmaterial synthesis, proteomics, and tools for research in life sciencesand forensic science.

What is claimed is:
 1. An active matrix electro-wetting on dielectric(AM-EWOD) device comprising: a plurality of array elements arranged inan array of rows and columns, each of the array elements including arrayelement circuitry, an element electrode, and a reference electrode;wherein the array element circuitry comprises an actuation circuit thatapplies actuation voltages to the element and reference electrodes foractuating the array element, and an impedance sensor circuit that sensesimpedance at the array element electrode to determine a droplet propertyat the array element; and wherein the actuation circuitry comprises amemory part including a memory capacitor for storing voltage datacorresponding to either an actuated state or an unactuated state of thearray element, and an input signal applied to the memory capacitoreffects an operation of the impedance sensor circuit; the actuationcircuit comprising a first transistor, the memory capacitor, and asecond transistor; wherein: a drain of the first transistor is connectedto an input DATA line, a gate of the first transistor is connected to aninput ENABLE line, and a source of the first transistor is connected toa gate of the second transistor; the memory capacitor is connectedbetween the gate of the second transistor and a row select line; and adrain of the second transistor is connected to an input actuationsignal, and a source of the second transistor is connected to theimpedance sensor circuit and the element electrode.
 2. The AM-EWODdevice of claim 1, further comprising a row select line that providesthe input signal to the memory capacitor, and the input signal from therow select line to the memory capacitor effects the operation of theimpedance sensor circuit by operating so as to isolate the array elementfrom the actuation voltage during the operation of the impedance sensorcircuit.
 3. The AM-EWOD device of claim 2, further comprising atransistor connected between the actuation voltage and the elementelectrode and whose gate is connected to the memory capacitor, whereinthe input signal from the row select line to the memory capacitoroperates to turn off the transistor to isolate the array element fromthe actuation voltage during the operation of the impedance sensorcircuit.
 4. The AM-EWOD device of claim 3, wherein the transistorconnected between the memory capacitor and the impedance sensor circuitis a p-type transistor.
 5. The AM-EWOD device of claim 1, wherein theimpedance sensor circuit comprises a reference capacitor and a sensorcapacitor, and the impedance sensor circuit operates by forming apotential divider circuit comprising the reference capacitor and thesensor capacitor, and an electrical load present at the array elementelectrode.
 6. The AM-EWOD device of claim 5, wherein the memorycapacitor acts as the reference capacitor.
 7. The AM-EWOD device ofclaim 5, further comprising a row select line that provides an inputsignal to the reference capacitor, wherein the input signal from the rowselect line to the reference capacitor effects an operation of theimpedance sensor circuit by perturbing a potential at the array elementas part of operating the impedance sensor circuit, and the dropletproperty is determined based on an amount of change of the potential atthe array element.
 8. The AM-EWOD device of claim 7, wherein thetransistor connected between the memory capacitor and the impedancesensor circuit is an n-type transistor, and further comprising anotherrow select line that provides an input signal to the n-type transistor,wherein: the row select line is connected to the reference capacitor; aninput signal from the row select line to the reference capacitorperturbs the potential at the array element as part of operating theimpedance sensor circuit; and an input signal from the another rowselect line to the n-type transistor is an inverse input of the inputsignal from the row select line to the reference capacitor.
 9. TheAM-EWOD device of claim 5, wherein the impedance sensor circuit furthercomprises a capacitor connected between the element electrode and a gateof a sensing transistor, and the impedance at the array element issensed based on a change in a sensing voltage that passes through thecapacitor to the gate of the sensing transistor.
 10. The AM-EWOD deviceof claim 9, further comprising a common addressing line that isconnected to each of the two transistors for setting the sensing voltageat the gate of the sensing transistor.
 11. The AM-EWOD device of claim9, further comprising a sensor input line over which an actuation signalis provided for actuating the impedance sensor circuit, and the memorycapacitor operates as the reference capacitor in the operation of theimpedance sensor circuit.
 12. The AM-EWOD device of claim 9, furthercomprising a sensor input line over which an actuation signal isprovided for actuating the impedance sensor circuit, wherein: the memorycapacitor operates as the reference capacitor in the operation of theimpedance sensor circuit; and the actuation voltage for actuating thearray element also is inputted to the one of the transistors of theimpedance sensor circuit for resetting the sensing voltage.